1. Field of the Invention
The invention relates generally to the field of signal generators for generating and transmitting tone signals, and more specifically to tone signal generators which generate and transmit tone signals in digital form. The signal generator may be expeditiously used to iteratively generate digital tone words of a multiplicity of frequencies in a number of different cadences and to transmit them in defined sequential time slots for use by a telephone switching system.
2. Description of the Prior Art
In recent years, telephony systems have undergone a number of substantial changes. Although they once were entirely analog, dedicating an entire path through the switch at the central office to one telephone call, telephony systems have become almost completely digital and multiplex a number of calls onto the same lines. The telephony system includes converters for converting signals representing voice or other subscriber information between analog and digital form. The digital signals are coupled through the switch under control of digital computers. Digitizing the voice signals allows signals from several calls to be multiplexed onto the same lines, in different time slots, thereby reducing the number of lines that are required in the telephony system. Digitizing the voice signals also allows the switch to be comprised of conventional digital memories and multiplexers under control of digital computers, rather than prior electro-mechanical relays, thereby significantly enhancing the reliability of the system.
A typical telephony system includes a "tone plant" which generates a number of tone signals having precise predetermined frequencies or combinations of such frequencies. These tone signals are used for a number of purposes, including signalling to other central offices over trunk lines, internal control, and signalling ringback tones, receiver off-hook tones, and the like over the subscriber lines. Presently, in most known telephony systems, the tone plant generates these precise tone signals in analog form, and then converts them to digital form for transfer through the switch to the required trunk or subscriber line. This requires the tone plant have a separate tone generator for each tone that is generated, and an analog-to-digital converter for each tone. The analog tone generator requires circuit elements such as capacitors and inductors of predetermined values for each tone frequency. Thus, if it is desired to change the frequencies, it would be necessary to change these components, which can be a laborious task. Furthermore, as in other analog systems, the frequencies generated by the analog tone generators can vary because of power or temperature changes, or merely with the aging of the components.
Furthermore, as has been mentioned, some signalling of the tone signals that are required, for example, for signalling over trunk lines, are combinations of two of the precise frequencies by the tone plant. The telephony system thus must have a circuit for combining selected analog tone signals to generate the required signalling tone.
The aforementioned U.S. patent application Ser. No. 06/497,667 describes a digital transceiver, a portion of which generates tone signals in digital form. The tone signal generator portion of the transceiver maintains a table of amplitude values, in digital word form, at specified equi-distant intervals over one wavelength of a normalized sinusoidal signal. In response to timing and frequency control signals from the telephony system, the tone generator iteratively selects an amplitude value from the amplitude table in response to the frequency control signals and to the number of timing signals that have been received from the beginning of the transmission of the tone. Specifically, the tone generator maintains a pointer into the amplitude table which is incremented each time the table is accessed by a frequency identification value proportional to the frequency being generated. The frequency identification values are chosen so that the tone generator will step through the table at a faster rate if it is generating a tone signal of a higher frequency than if it were generating a tone signal of a lower frequency.
Since in most known telephony systems the analog voice and tone signal are converted to digital form according to a logarithmic conversion protocol, termed a "u-255" standard, the tone signal generator then converts the digital word from the amplitude table, which is termed a "linear value", into the logarithmic value. To do so, it maintains a "PCM table" (PCM standing for "pulse code modulation"). Each digital amplitude word in the amplitude table identifies a digital tone word in the PCM table, the digital tone word in the PCM table representing the conversion of the digital amplitude word by way of the u-255 standard. The digital tone word output from the PCM table is then transmitted as the output of the tone signal generator.